Synthetic markers are used to provide visual cues that are easy to detect for computer vision systems. Such markers can be used in robotics, automation and media-oriented applications such as advertising, coupon redemption, nutrition information on packaging, etc to encode a symbolic label (i.e. a number). An example of a synthetic marker is a bar-code. Other examples of general purpose synthetic markers and related encoding schemes are the US Postal Service's Maxi-Code™ as illustrated in FIG. 1a, which is used to encode shipping information in packages. QuickResponse™ and Data Matrix™ as illustrated in FIG. 1b is used for carrying information for part labeling.
A sub-class of synthetic markers are fiducial or fiduciary markers. Fiducial markers are synthetic markers which are better adapted to provide visual cues for positioning an object in space. These markers can thus be used to provide position estimates for robotics applications. They can also be used to label individual objects for their manipulation or for scene augmentation purposes in the field of augmented reality.
Some encoding methods used to create synthetic markers involve error correction schemes which allow for some recovery of encoded information in case part of the information retrieved from the marker is corrupted. Such encoding methods tend to produce markers which are however limited when employed as fiducial markers. This is in part because such markers can be hard to detect in a large field of view or at large viewing distances due to perspective distortion or atmospheric scattering for example. These markers are further limited to a relatively small detection range to ensure proper detection and decoding of the marker's information payload.
Examples of fiducial markers used in augmented reality applications are ARToolKit™ and ARTag™ markers which are illustrated in FIG. 1c and FIG. 1d, respectively. Theses markers are made of black and white (or bi-tonal) square or geometric patterns, which encode information payload such as multi-bit data.
In deteriorated viewing conditions (i.e. Large viewing distances, camera noise, environmental or other factors) the geometrical pattern of these markers eventually becomes ambiguous and although their landmark is possibly still observable, their information payload is either extractable only with great difficult or not at all.